Not quite. They’re developed by teams of subject matter experts (SMEs). Some of these SMEs work for MIcrosoft, and some come from the broader IT community. The exam development process is too long and boring to talk about in depth; there are several phases to it, starting with “hey, kids, let’s put on an exam” and culminating with the rollout of the finished exam.
Along the way, there are usually beta opportunities, where the nearly-finished test is made available to a limited pool of test users who can give feedback to the test development team (i.e. “hey, question 45 is too vague!” or “there’s no correct answer to question 15.”) For the first time that I know of, Microsoft has opened a public call for SMEs who want to volunteer to work on exam development and to get early access to exam betas. If you’re at all interested, visit this site to get directions for signing up. (Hint: it involves Microsoft’s Connect web site!)